1. Field of the Invention
The present invention concerns a method for correction of B0 field drift in a temperature map exposure by means of magnetic resonance tomography. The invention furthermore concerns a device to implement such a method.
2. Description of the Prior Art
An image representation of an examination subject—in particular the body of a patient—that graphically reproduces the spatial distribution of the temperature or of a temperature difference within the examination subject is designated as a temperature map. Such a temperature map can be acquired by means of a magnetic resonance (MR) tomography apparatus scanner.
The presentation of temperature maps that show the time change of the temperature distribution within a patient body is useful in the context of tumor ablation in interventional radiology. The tumor tissue to be removed is heated (and thus killed) non-invasively by the application of focused ultrasound or by the application of laser light or radio waves by means of a probe inserted into the patient's body.
In order to be able to continuously monitor the temperature in the patient body during such a procedure (more precisely, in the region of the tissue to be ablated), the procedure typically occurs either directly in an MR scanner or in immediate proximity thereto. The temperature monitoring ensues by generating temperature maps of the patient's body continuously or at regular intervals over a longer time span, typically 30 to 120 minutes.
The nuclear magnetic resonance frequency of hydrogen atoms (protons) in a water molecule (and thus in particular in the water molecules of the patient's body) exhibit a characteristic temperature dependency. This physical effect is typically utilized for non-invasive temperature determination by means of magnetic resonance tomography. As a result of this temperature dependency, temperature differences in the patient's body are manifested by a spatial variation of the phase of the magnetic resonance signal, that is represented in the form of complex numbers. Information about the temperature distribution in the patient body at the point in time of acquisition therefore can be obtained from an image known as a phase image, that reproduces this phase with spatial resolution.
Phase images that were acquired at the points in time to be compared are typically subtracted pixel-by-pixel from one another to determine the (spatially resolved) temperature difference in the patient body between different acquisition points in time. In interventional radiology, in particular a “heated” phase image (therefore a phase image acquired at a point in time during the procedure) is subtracted from an “unheated” phase image (i.e. a phase image acquired before the beginning of the procedure). The temperature difference between the two acquisition points in time is then calculated from the resulting difference image for each pixel. The generation of temperature maps by means of MR tomography is designated as (MR) thermometry.
The precision of a temperature map generated in such a manner, in particular the precision with which the temperature difference between two acquisition points in time at a specific location of the patient body can be determined from such a temperature map, is typically impaired by the B0 field drift. The B0 field drift is a slow variation of the basic magnetic field (B0 field) of the MR scanner that is used to generate the temperature map.
Variation of the B0 field also disadvantageously leads to a change of the proton resonance frequency to which the acquisition of a temperature map is tied (in terms of measurement). Like temperature changes in the patient body, variations of the B0 field thus directly affect the phase images, but temperature-dependent phase changes and B0 field-dependent phase changes cannot be differentiated from each other without additional measures. An error in the resulting temperature map that normally grows with increasing distance between the acquisition points in time thus arises due to the B0 field drift.
To correct the influence of the B0 field drift in temperature maps, it has previously been typical to identify spatial regions of the phase images to be compared—in which spatial regions no temperature change is to be expected between the acquisition points in time—and to use such regions as correction regions.
For B0 field drift correction, the correction regions in the phase images to be compared are calibrated to one another. In other words, one of the phase images (normally the phase image acquired later) to be compared is corrected such that the correction region of the corrected phase image coincides with the correction region of the other comparison image with regard to the phase values, so a temperature difference of zero results.
To produce such a correction region, usually a magnetic resonance phantom filled with water at room temperature is placed in the examination volume while each phase image is being acquired. Alternatively, a region of the patient's body in which no heating or cooling between the acquisition points in time is expected can be defined as a correction region. The correction regions are to be manually defined by the user in any case.
Both known methods for correction of the influence of the B0 field drift are relatively error-prone. In particular, displacement of the phantom or (if the correction region was defined in the patient's body) a change of the patient position between the acquisition points in time can lead to a greater error of the resulting temperature map as a resulting of incorrect B0 drift correction.
An error of the B0 drift correction can also result from the fact that an unintended temperature change nevertheless occurs in the correction region between the acquisition points in time. Such errors in particular occur when the correction region is selected in the patient's body.